Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic elements written at least in part by a current driven through the magnetic element.
For example, FIG. 1 depicts a conventional magnetic element 10, which may be a conventional magnetic tunneling junction (MTJ) or a conventional spin valve. The conventional magnetic element 10 may be used in a conventional magnetic memory, such as a conventional STT-RAM. In memory applications, the conventional magnetic element 10 is typically a conventional MTJ. The conventional MTJ 10 typically resides on a substrate (not shown), uses conventional seed layer(s) 11 and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional barrier or spacer layer 16, a conventional free layer 18, and a conventional capping layer 20.
The spacer layer 16 is nonmagnetic. The spacer layer 16 may be a tunneling barrier layer, for example a thin insulator, or a conductor. The conventional capping layer 20 is typically used to provide protection for the underlying layers 11, 12, 14, 16, and 18. The conventional seed layer(s) 11 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 12, having a desired crystal structure.
The conventional pinned layer 14 and the conventional free layer 18 are magnetic. The magnetization 15 of the conventional pinned layer 14 is fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layer 12. Although depicted as a simple (single) layer, the conventional pinned layer 14 may include multiple layers. For example, the conventional pinned layer 14 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. In such a synthetic layer, multiple magnetic layers interleaved with a thin layer of Ru may be used. Further, other versions of the conventional magnetic element 10 might include an additional pinned layer (not shown) separated from the free layer 18 by an additional nonmagnetic barrier or conductive layer (not shown).
The conventional free layer 18 has a changeable magnetization 19. Although depicted as a simple layer, the conventional free layer 18 may also include multiple layers. For example, the conventional free layer 18 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru.
The conventional magnetic element 10 may use spin transfer torque to write to the conventional magnetic element 10. In particularly, spin transfer torque rotates the magnetization 19 of the conventional free layer 18 to one of the two directions along its easy axis. When a write current is passed through the conventional magnetic element 10 perpendicular to the plane of the layers, electrons may be spin polarized by the conventional pinned layer 14. The spin transfer torque on the magnetization 19 of the conventional free layer 18 may be given by: T˜I*MFL*MPL*sin(θ), where θ is the angle between the magnetization 19 of the free layer 18 and the pinned layer magnetization 15 with large enough current to generate adequate torque to switch the conventional free layer 18. With a sufficient current, the conventional free layer 10 may be written to the desired state.
Applications for the conventional magnetic element 10 such as STT-RAM require the conventional magnetic element 10 to be thermally stable. Stated differently, the conventional magnetic element 10 has sufficient thermal stability that the magnetization 19 of the conventional free layer 18 is not be switched by thermal fluctuations. In order for this to occur, the conventional free layer 18 has a magnetic anisotropy energy sufficient to retain the magnetization 19 in the direction it was written. If the magnetic anisotropy energy is too small the magnetization can be rotated to other directions by random thermal fluctuations, and the stored information is lost.
The magnetic anisotropy energy of a magnetic element with a uniaxial magnetic anisotropy can be given by:Eb=KuV  (1)
where Ku is the magnetic anisotropy constant and V is the volume of the element. The thermal stability factor of such a magnetic element is defined as:
                    Δ        =                                            K              u                        ⁢            V                                              k              B                        ⁢            T                                              (        2        )            
where kB is Boltzman constant and T is temperature. The kBT is the energy of thermal fluctuations. To prevent the conventional magnetic element 10 from being switched by thermal fluctuations within a given period of time, Δ has to be larger than a particular value set by design considerations. For example, to keep the magnetization 19 in the switched state for ten years, Δ is above approximately forty.
When the conventional magnetic element 10 has dimensions in the deep sub-micron scale, the volume V of the element is very small. To have a large enough Δ, therefore, the anisotropy constant Ku is very large. Factors such as intrinsic or crystalline anisotropy and shape anisotropy contribute to the anisotropy constant. Thus, these properties are tailored to obtain the desired Δ, as well as other magnetic characteristics of the conventional magnetic element 10. Many conventional magnetic devices typically use materials with fairly small intrinsic magnetic anisotropy constant to obtain desired magnetic properties. Consequently, a common method of increasing Ku is to pattern the conventional magnetic element 10, particularly the conventional free layer 18, into elongated shapes. Such shapes introduce a shape magnetic anisotropy. Such a shape is depicted in FIG. 1. The easy axis of the conventional free layer 18 lies along the long axis of the ellipse into which the conventional magnetic element 10 has been patterned. Thus, the magnetization 19 of the conventional free layer 18 may remain stable in the direction shown in FIG. 1.
FIG. 2 depicts the shape of the conventional magnetic element 10 under various conditions. Stability of the magnetization 19 of the conventional free layer 18 due to shape anisotropy may be explained with reference to FIG. 2. As discussed above, the majority of the anisotropy of the conventional free layer 18 comes from its shape anisotropy. When the magnetization 19 of the element points to a certain direction, for example at an angle θ from the long axis of the ellipse (the easy axis), positive and negative magnetic charges are produced at the edges of the ellipse by the magnetization. The magnetic charges induce a demagnetizing magnetic field 20 pointing substantially opposite to the magnetization 19. The magnetostatic energy density caused by this field is:Es=−M·H  (3)
Where M and H are vectors. As a result, the magnetostatic energy density due to the shape of the conventional magnetic element 10 is:Es=Ks sin2θ  (4)
where Ks is the shape induced uniaxial anisotropy constant. If the magnetization 19′ is perpendicular to the easy axis, the magnetostatic energy of the cell is largest. As can be seen in the free layer 18′ of FIG. 2, this occurs because the positive and negative magnetic charges are closer together and the demagnetizing field 22 is highest. When the magnetization 19″ is aligned with the long, or easy, axis, the charges on the surface are farther apart. Thus, the demagnetizing field 22″ is smaller and the magnetostatic energy is smallest. This can be seen in the free layer 18″ of FIG. 2. The energy barrier height between the two easy directions (θ=0° and 180°) then equals to KsV—the energy difference between high-energy state (magnetization 19′ along the short axis) and low-energy state (magnetization 19″ along the long axis). Thus, the shape anisotropy may provide thermal stability.
Although utilizing a shape anisotropy may provide thermal stability, this approach has drawbacks. In particular, photolithography is typically used to define the shape of the conventional magnetic element 10. Conventional techniques make defining the shape of the conventional magnetic element difficult. Further, as the density of STT-RAM memories increase and the size of the conventional magnetic element 10 decreases, this conventional approach becomes increasingly difficult.
Accordingly, what is needed is a method and system that may improve the thermal stability of the spin transfer torque based memories. The method and system address such a need.